It was found that the Xen hypervisor x86 CPU emulator implementation did not correctly handle certain instructions with segment overrides, potentially resulting in a memory corruption. A malicious guest user could use this flaw to read arbitrary data relating to other guests, cause a denial of service on the host, or potentially escalate their privileges on the host.

ISSUE DESCRIPTION
=================
Instructions with register operands ignore eventual segment overrides
encoded for them. Due to an insufficiently conditional assignment such
a bogus segment override can, however, corrupt a pointer used
subsequently to store the result of the instruction.
IMPACT
======
A malicious guest might be able to read sensitive data relating to
other guests, or to cause denial of service on the host. Arbitrary code
execution, and therefore privilege escalation, cannot be excluded.
VULNERABLE SYSTEMS
==================
Xen 3.2.x and later are vulnerable.
Xen 3.1.x and earlier have not been inspected.
Only x86 systems are vulnerable. ARM systems are not vulnerable.
MITIGATION
==========
There is no mitigation available for this issue.
RESOLUTION
==========
Applying the appropriate attached patch resolves this issue.
xsa123.patch xen-unstable, Xen 4.5.x, Xen 4.4.x
xsa123-4.3-4.2.patch Xen 4.3.x, Xen 4.2.x
$ sha256sum xsa123*.patch
c65ff7e169416b8c8aef93f2f1b4c259f123f244cdb3ab7a1f4fe714000752ff xsa123-4.3-4.2.patch
6f1587f9e0a4b9325c75c004b0069b5d5303a725cf7dd8def18c01b9d0aa8912 xsa123.patch